The present invention relates to a three-beam type optical data disk player with a servo circuit in which three optical beams including a main optical beam are used, and the main optical beam is caused to correctly follow the track on the disk to read data recorded on the disk.
In order to correctly read data from a disk with an optical data disk player, it is essential to cause the reading light beam to correctly follow the track on the disk. To do so either a single-beam system can be employed in which a single reading light beam is used, or a so-called three-beam system can be employed in which two auxiliary light beams are provided which are directed onto points on the disk before and after the main light beam.
FIG. 1 shows an example of a conventional three-beam type optical data disk player. In FIG. 1, reference numeral 11 designates a track on a disk; 12, a main beam for reading the data recorded on the disk; 13, a front beam directed in front of the main beam 12 for generating a signal utilized to form a radial error signal used to control the main beam 12 to follow the track 11; 14, a rear beam directed to the rear of the main beam and used for generating a signal utilized to form a radial error signal; 15, a main beam preamplifier for amplifying data signals read by the main beam 12; 16, a front beam preamplifier for extracting a low frequency component from the signal detected by the front beam and amplifying that component; and 17, a rear beam preamplifier for extracting a low frequency component from the signal detected by the rear beam and amplifying that component.
Further in FIG. 1, reference numeral 18 designates a differential amplifier for obtaining the difference between the outputs of the front beam preamplifier and the rear beam preamplifier so as to form a so-called radial error signal; 19, a radial servo receiving the radial error signal from the differential amplifier 18 and driving a radial actuator 20 to cause the main beam to follow the central line of the track 11; 20, the aformentioned radial actuator for causing the main beam 12 to follow the central line of the track 11; and 21, a servo gain control circuit for, in response to the output of the front beam preamplifier, controlling the servo gain of the radial servo circuit 19.
The operation of the disk player shown in FIG. 1 will be described with reference to the operating waveform diagram of FIGS. 2A through 2E. A data signal recorded on the track 11 is read by the main beam 12 and amplified by the main beam preamplifier 15, then outputted as a high frequency signal (RF signal) b as indicated in FIG. 2B. The high frequency signal is converted into an audio signal by a demodulating circuit, error correcting circuit, and D/A converter (not shown).
Further, the front beam preamplifier and the rear beam preamplifier extract the low frequency components from the signals read with the front beam 13 and the rear beam 14 and output those components as radial error detection signals a and c, as shown in FIGS. 2A and 2C, respectively. The differential amplifier 18 obtains the difference between the radial error detection signals a and c to form a radial error signal d, as shown in FIG. 2D. The radial error signal d is applied to the radial servo circuit 19. In response to the radial error signal d, the radial servo circuit 19 drives the radial actuator 20 so that the main beam 12 is moved, that is, made to follow the central line of the track 11.
At the same time, the radial error detection signal a of the front beam preamplifier 16 is applied to the servo gain control circuit 21. The servo gain control circuit 21 produces a gain control signal e which, when the radial error detection signal a exceeds a predetermined detection level DLH or DLL, as shown in FIG. 2A, is at a second gain setting level eH, namely, a high level for a certain period of time T, and is at a first gain setting level eL, namely, a low level, in the other cases. The gain control signal e thus produced is applied to the radial servo circuit 19. When the radial servo circuit 19 receives the first gain setting level eL from the servo gain control circuit 21, the servo gain of the circuit 19 is set to a first servo gain value, namely, a predetermined low gain. In contrast, when the radial servo circuit 19 receives the second gain setting level eH from the servo gain control circuit 21, the servo gain of the circuit 19 is set to a second servo gain value, higher than the first servo gain value, so that the servo control capability of the circuit 19 is increased.
In the case where no external disturbance exists and the disk has no local defect, only residual errors such as errors due to disk eccentricity occur. Therefore, in this case, the output a read with the front beam 13 never exceeds the detection level DLH or DLL. Accordingly, the servo gain control circuit 19 continuously outputs the first gain setting level eL (low level) so that the servo gain of the radial servo circuit is set to the first servo gain value.
When an external disturbance such as vibration is applied to the optical data disk player, the three beams are moved in the same direction. It is assumed that the main beam 12 is moved towards the left in FIG. 1. In this case, the part of the front beam on the track 11 is increased, while the part of the rear beam on the track 11 is decreased. The part a(n) of the radial error detection signal a in FIG. 2A, and the part c(n) of the radial error detection signal c in FIG. 2C indicate radial error detection signals caused by such an external disturbance. In this case, the level of the radial error detection signal a outputted by the front beam preamplifier 16 exceeds the detection levels DLH and DLL, and therefore the servo gain control circuit 21 outputs the second gain setting level eH(n) (high level), as shown in FIG. 2E. The second gain setting level, applied to the radial servo circuit 19, cause the servo gain of the latter to be set to the predetermined second servo gain value for the predetermined period of time T, and thus the external disturbance eliminating capability is increased. As a result, the radial actuator is driven so as to return the main beam 12 to the central line of the track 11.
In the case where the disk has a local defect, the front beam 13, the main beam 12 and the rear beam 14 pass through the local defect in the stated order. Therefore, as indicated at a(d) in FIG. 2A, at b(d) in FIG. 2B, and at c(d) in FIG. 2C, first the radial error detection signal a of the front beam preamplifier 16 becomes irregular, then the high frequency signal b of the main beam preamplifier 15 drops out, and finally the output of the rear beam preamplifier 18 becomes irregular.
In the case, the radial error detection signal a of the front beam preamplifier 16 exceeds the detection level DLL. Therefore, as in the case of the abovedescribed external disturbance, the servo gain control circuit 21 outputs the second gain setting level eH(d) so that the radial servo circuit 19 drives the radial actuator 20 with the servo gain increased to the second servo gain for the predetermined period of time T.
As is apparent from the above description, the conventional optical data disk player employs a system whereby the servo gain switching control signal is produced by detecting when the radial error detection signal is produced by detecting when the radial error detection signal a of the front beam preamplifier 16 exceeds the predetermined detection levels. As a consequence, in the conventional optical data disk player, it is impossible to distinguish a radial error detection signal disturbance due to a local disk effect from a disturbance due to an external disturbance, and accordingly in the case of a local defect, as in the case of the external disturbance, the servo gain of the radial servo circuit 19 is increased to the second servo gain. However, the radial error detection signal disturbance due to the local defect occurs completely irrespective of the position of the beam on the track. Accordingly, although the main beam 12 follows the track 11 correctly, the radial actuator is driven in the wrong direction because the servo gain of the radial servo circuit has been increased to the second servo gain as described above. That is, the main beam 12 is caused to jump the track.